The magnetic recording medium is widely used, for example, as a sound recording tape, a video tape or a floppy disk. The magnetic recording medium comprises a non-magnetic support having provided thereon a magnetic layer composed of a binder having dispersed therein ferromagnetic powder.
The magnetic recording medium must have various characteristics on a high level, such as electromagnetic conversion characteristics, running durability and running performance. More specifically, an audio tape for playing back a recorded music must have an ability to reproduce the original sound to a higher extent. A video tape must have a superior electromagnetic conversion characteristics such as excellent ability to reproduce the original image.
In addition to such superior electromagnetic conversion characteristics, the magnetic recording medium is required to have good running durability as described above. In order to achieve good running durability, the magnetic layer usually contains an abrasive and a lubricant.
However, for achieving good running durability with the use of an abrasive, the abrasive needs to be added in an increased amount to a certain degree and accordingly, the content of the ferromagnetic powder is reduced. Further, when an abrasive having a large particle size is used to achieve excellent running property, the abrasive is prone to project excessively on the surface of the magnetic layer. As a result, the improvement in running durability by the use of an abrasive is associated with a problem that the abovedescribed electromagnetic conversion characteristics are deteriorated in many cases.
Furthermore, when the above-described running durability is improved by the use of a lubricant, the lubricant needs to be added in a large amount and accordingly, the binder is readily plasticized, thereby the durability of the magnetic layer is prone to be reduced.
The binder as a main component of the magnetic layer plays an important role in achieving improvement in the above-described durability and electromagnetic conversion characteristics. However, with a conventionally used binder such as a vinyl chloride resin, a cellulose resin, a urethane resin or an acrylic resin, the magnetic layer can have inferior abrasion resistance and involves a problem because it contaminates members in the running system for the magnetic tape.
In order to overcome such problems, a hard binder is used to increase the hardness of the magnetic layer. However, by increasing the hardness of the magnetic layer, the magnetic layer becomes brittle conspicuously to cause problems such as occurrence of the dropout upon abutting against the magnetic head or deterioration in the still characteristics.
JP-A-62-134819 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-62-208423 disclose another countermeasure, in which the magnetic recording medium comprises a hydroxyl group not bonded directly to the main chain (bonded through an alkyl group or an alkylester group) and a vinyl chloride copolymer having a polar group and does not contain vinyl alcohol as the copolymer component. The vinyl alcohol as the copolymer component is produced through saponification and the polymer containing the vinyl alcohol hardly undergoes changes by aging, accordingly, the magnetic recording medium composed thereof is hardly changed by aging, has superior durability, exhibits excellent dispersibility of ferromagnetic powder due to the presence of a hydroxyl group and a polar group and is favored with good electromagnetic conversion characteristics.
However, in the above-described magnetic recording medium, the magnetic layer is not high in hardness and the surface of the magnetic layer is insufficient in smoothness and accordingly, the durability and the electromagnetic conversion characteristics cannot be sufficiently superior. As a result, it cannot be an adequately superior magnetic recording medium as a tape required to have extremely high smoothness and excellent electromagnetic conversion characteristics, for example, for S-VHS and 8 mm video.
Further, JP-B-63-55549 (the term "JP-B" as used herein means an "examined Japanese patent publication") describes the use of a modified polyurethane resin having a polyvalent OH group and SO.sub.3 M (M represents an alkali metal ion or a quaternary ammonium ion) as a magnetic coating composition. By using such a binder, the surface gloss, blocking property and adhesion are improved.
With respect to the binder composed of polyurethane, JP-B-58-41565 describes a polyurethane binder having --SO.sub.3 M using polyesterpolyol as polyol, JP-A-59-198530 describes a polyurethane binder obtained from polycarbonate having a hydroxy end group and diisocyanate and JP-A-62-201918 describes a polyurethane binder starting from carboxyl group-containing polycaprolactone. Further, JP-A-61-190717 describes a polyurethane binder starting from carboxyl group-containing polyether.
Although such a binder composed of polyurethane exerts excellent characteristics inherent to polyurethane, it is insufficient with respect to the dispersibility of ferromagnetic powder and the long-term storage stability and also it is inadequate in durability under conditions over a wide range of temperatures and humidities.
JP-A-61-104325,JP-A-62-121922 and JP-A-3-201211 describe examples of polyurethane containing polyether/polyesterpolyol. JP-A-61-104325 describes a urethane synthesized from diisocyanate and polyetheresterpolyol which is obtained by condensing (1) PTMG (HO(C.sub.2 H.sub.4 --O--).sub.n --H (1.ltoreq.n.ltoreq.50)), (2) diol having EO (ethylene oxide) and PO (propylene oxide) added to a phenolic OH and having a molecular weight of 1,000 or less and (3) an aromatic dicarboxylic acid (isophthalic acid). In other words, the polyether segment is readily associated because it has a blocking property but the polyester segment is hardly associated due to the lack of blocking property. When n is 1,the product is polyesterpolyol. In examples thereof, the polyether has a molecular weight of 650 or 1,000 and the polyether segment is readily associated due to the presence of blocking property but the polyester segment is not associated due to the lack of blocking property.
JP-A-62-121922 describes polyurethane having the same structure as that disclosed in JP-A-61-104325Further, JP-A-3-201211 describes polyurethane having a structure as blocking polyurethane comprising polyetherurethane having an OH end group and polyesterurethane connected to each other through diisocyanate and the polyesterurethane described is mainly polyesters having an ether bond therein. In the example of this technique, polycaprolactone polyester is used as polyesterpolyurethane. Accordingly, the object of this technique is to prevent the hydrolyzability and also in examples, improvement in the hydrolyzation durability is emphasized as an effect. The aromatic carboxylic acid used as the polyester component is enumeratively described in the detailed description, however, the feature of the invention doe not reside in achieving high glass transition point Tg.
The polyetherpolyurethane uses PEG (polyethylene glycol), PPG (polypropylene glycol) or PTMG (polytetramethylene glycol) as the starting materials for a long-chained polyol and it is excellent in the hydrolyzation durability. However, it is deficient in that the thermal deformation temperature (or Tg) is low and the mechanical strength is poor. JP-A-3-201211 discloses polyurethane using polyetherpolyol or polyesterpolyol as a long-chained polyol, however, the polyesterpolyol used is an aliphatic polyesterpolyol, therefore, it is insufficient in terms of the Tg and the mechanical strength and it is in need of improvement with respect to the physical properties. JP-A-3-201211 has a main object to improve the hydrolyzation durability of polyurethane and the polyurethane, when used as it is, cannot satisfy overall characteristics of the magnetic recording medium and is hardly durable in practical use.
Polyurethane using a long-chained diol obtained by esterifying polyetherpolyol with an aromatic dicarboxylic acid may be used so as to improve the heat durability and the Tg of polyetherurethane. The polyether segment thereof has a blocking property but the polyester segment does not have. According to this method, although the heat durability and the Tg may be improved, the polyurethane produced has a reduced ultimate elongation and is brittle, thus it provides no durability to the magnetic recording medium and it is inadequate in practical use. In other words, due to the brittle magnetic layer, the magnetic layer may fall as a debris at the edge portion of the tape during VTR running to cause dropout (DO) or the magnetic layer may crack at the tape edge portion during slitting of the tape to cause falling of powder or DO.
The present inventors have conducted intensive investigations on the structure of polyurethane and the kinds of ferromagnetic powder and as a result, they have found that a magnetic recording medium extremely excellent in electromagnetic conversion characteristics, durability and long-term storage stability can be obtained by a combination use of specific ferromagnetic powder and specific polyurethane.